Charged particle beam apparatuses are used in a plurality of industrial fields. Inspection of semiconductor devices during manufacturing, exposure systems for lithography, detecting devices and testing systems are only some of these fields.
In general, there is a high demand for structuring and inspecting specimens within the micrometer or nanometer scale. On such a small scale, process control, inspection or structuring is often done with charged particle beams, e.g. electron beams, which are generated and focused in charged particle beam devices such as electron microscopes or electron beam pattern generators. Charged particle beams offer superior spatial resolution compared to, e.g. photon beams due to their short wavelengths.
Generally, charged particle beam devices are operated under vacuum conditions to avoid, e.g. ionization of surrounding gases. In spite of that, electrons impinging on component surfaces of the device, like extractors, anodes, apertures or the chamber wall result in an emission of contaminants. Thus, a shower of residual gas is generated. The residual gas contains molecules which can be hit by electrons. Thereby, ions, ionized molecules and other particles can be created. In the case of ions and ionized molecules having a charge which is opposite to the charge of the charged particles emitted by an emitter, the ions and ionized molecules in the residual gas are accelerated towards the emitter. As a result, the emitter can be mechanically deformed from the impingement of the ions and ionized molecules or these particles can be deposited on the emitter. Thus, emitter noise is introduced.
According to a known solution, a pretreatment is conducted. A respective apparatus, shown in FIG. 4, will be described in the following. In FIG. 4, an emitting unit comprising a wire 12 and a field emitter 14 is shown. If high voltages are applied between the emitter and the extractor 8, the field emitter emits charged particles, e.g. electrons along optical axis 1. Further, an anode 6 and aperture 7 are provided. These devices are used to avoid a widespread emission, an acceleration of the charged particles and a beam shaping. Further, condenser lens 4 can be used to image the electron source or a beam crossover acting as a virtual electron source.
Before operating the charged particle beam device, it has to be evacuated. After a certain vacuum level has been reached, an electron gun 42 with a high current floods the chamber with electrons. The emitted electrons impinging on the walls or other surfaces of parts of the column and additional heat detach volatile molecules from the surfaces of the column. Thereby, residual gas is created. The residual gas gets pumped out of the chamber by vacuum pumps.
As a result, within this cleaning step, residual gas in the form of molecules attached to column surfaces gets pumped out of the column before the intended use of the charged particle device starts. On the one hand, ions created during the cleaning step do not damage field emitter 14 and are hardly deposited thereon. On the other hand, the molecules and ions possibly damaging the field emitter during intended use are pumped out of the chamber.
However, the known solution is still not satisfactory.